    [Patent Literature 1] JP H07-106528 A (JP 2,929,909 B)
The nerve cells of a human brain (hereafter, referred to as “neuron”) have a recognizing ability and a judging ability. For example, a neuron is capable of taking the following action: when a pattern is repeatedly inputted, it stores the pattern; when another pattern is inputted subsequently, it recognizes whether another pattern subsequently inputted is identical with the pattern that is stored; and when another pattern is identical, it outputs a signal in response thereto. The more frequently identical information is repeatedly inputted to a neuron, the more readily the neuron stores the inputted identical information and the more readily the neuron responds when the identical information is inputted. This means that the neuron becomes more responsive to an input signal.
This action depends on the structure of the neuron. Specifically, a neuron is so structured that many synapses are connected and communication is carried out from a synapse corresponding to inputted information to the neuron. When inputted information is identical with stored information, the neuron outputs a signal to other neurons. The more frequently identical information is repeatedly inputted, the tighter the relation between a specific synapse and the neuron becomes. When repeatedly inputted information is inputted again, communication from a specific synapse to the neuron is facilitated and the neuron more easily outputs a signal to another neuron in response thereto.
It has been attempted to implement this function of the human brain with semiconductor. For example, Patent Literature 1 proposes a field-effect transistor forming a neuron element in which a neuron is materialized with semiconductor. With this field-effect transistor, the learning function of the neurons of brains and multi-input parallel processing can be implemented.
This field-effect transistor is formed using, for example, a p-type silicon substrate where a gate insulating film, a floating gate electrode, a ferroelectric film, and a multi-input gate electrode are formed in this order over a channel formed between a source region and a drain region.
The multi-input gate electrode of the field-effect transistor includes a plurality of separated electrodes. In some electrodes, which voltage is applied to, of the plurality of separate electrodes, the polarization occurs in a ferroelectric film provided between the multi-input gate electrode and the floating gate electrode; this accumulates electric charges. The amount of accumulated electric charges is increased with increase in the number of times of voltage application; the capacitance between the electrode to which voltage is applied and the floating gate electrode is increased with increase in the amount of electric charges.
The field-effect transistor is turned on when voltage is applied to the multi-input gate electrode, under the condition that: the potential ϕf generated to the floating gate electrode based on the electric charges accumulated in the ferroelectric film should become equal to or higher than threshold voltage Vth. The potential ϕf generated to the floating gate electrode takes a value corresponding to the total sum of the products of capacitances and applied voltages; each product is of (i) the capacitance in the place of the ferroelectric film between each of the separated electrodes in the multi-input gate electrode and the floating gate electrode and (ii) the applied voltage to each input gate electrode. As mentioned above, larger capacitance arises in the place where (I) voltage is applied to the multi-input gate electrode a larger number of times and (ii) a larger amount of electric charges are accumulated. Therefore, in a place where voltage was applied many times, a larger value is taken by the product of the capacitance and the applied voltage when the voltage is applied in next time.
Supposing a case where the number of times of voltage application is increased to the electrodes located in places corresponding to group information of the electrodes in the multi-input gate electrode. In such a case, when group information is inputted again and voltage is applied to the electrodes corresponding to the group information in the multi-input gate electrode, the field-effect transistor is thereby readily turned on. The field-effect transistor enables multi-input, stores multi-input corresponding to group information, and is readily turned on when group information is inputted. That is, the field-effect transistor is capable of recognizing group information and responding thereto.
However, the above field-effect transistor may respond and output a signal not only when voltage is applied to the electrodes corresponding to group information in the multi-input gate electrode but also when voltage is applied to other electrodes as well. That is, when voltage is applied to electrodes corresponding to group information, the total sum of the products of the capacitances corresponding to the electrodes and the applied voltages is increased and the field-effect transistor is readily turned on. Even when voltage is applied to electrodes other than the electrodes corresponding to the group information, the total sum is accordingly increased. This turns on the field-effect transistor; thus, the neuron element no longer responds only to a stored pattern.